Heretofore, the following properties have been required for a glass substrate to be used for display devices such as liquid crystal display devices, particularly for a glass substrate designed to have a thin film of a metal or oxide formed on their surface.
(1) It contains substantially no alkali metal ion (i.e. it is alkali-free glass).
If a glass substrate contains alkali metal oxides, alkali metal ions are likely to diffuse into the above thin film thereby to deteriorate the properties of the thin film.
(2) It has a high strain point, so that deformation of a glass substrate and shrinkage (heat shrinkage) accompanied by stabilization of the glass structure due to heating during the process for forming the thin film, can be suppressed to the minimum level.
(3) It has adequate chemical durability against various chemicals to be used for formation of semiconductors. Especially, it has the chemical durability against buffered hydrofluoric acid (BHF: hydrofluoric acid+ammonium fluoride) to be used for etching of SiOx or SiNx, chemicals containing hydrochloric acid to be used for etching of ITO (tin-doped indium oxide), various acids (e.g. nitric acid, sulfuric acid) to be used for etching of a metal electrode, or a resist-removing alkaline liquid.
(4) It has no defects (e.g. bubbles, stria, inclusions, pits or flaws) inside or on the surface.
In recent years, a glass satisfying the following properties as well as the above, has been desired.
(5) A display has been required to have its weight reduced, and glass itself has been also desired to be a glass having a low density.
(6) As a method for reducing the weight of a display, it has been desired to reduce the thickness of a glass substrate.
(7) In order to increase the high productivity or improve the thermal shock resistance by increasing the temperature rising or falling rate at the time of heat treatment for producing the liquid crystal display, a glass having a small linear expansion coefficient has been required.
(8) As liquid crystal TV sets are widely used and are made to have a large size, a glass substrate has been required to have a large area of 2 m square from 1 m square of a conventional glass substrate. When a display using such a large substrate is to be prepared, the glass substrate has been required to have a high Young's modulus so as to reduce deflection of the glass substrate by its own weight at the time of transportation.
(9) A glass substrate has been required to have a high strength so as not to be broken by an external force or shock exerted during the use of the liquid crystal display product.
As a method for forming glass, a float process has been widely used wherein glass is formed on molten tin, but in such a process, glass is exposed to a reducing atmosphere as an atmosphere of a mixture of nitrogen with hydrogen at a relatively high temperature (a temperature (T4) at which the glass viscosity is in the vicinity of log η=4 (poise)), whereby the glass surface is susceptible to a reducing effect. Such a reducing effect presents various adverse affects to the glass. For example, the interaction between S2− and Fe2+ which are produced by reduction of SO42− which is dissolved in glass as an impurity, causes coloration (amber coloration), or Fe ions in glass tend to be metallized and precipitated and thereby become starting points for devitrification or starting points for precipitated crystals at the time of immersion into BHF, thus leading to deterioration of the BHF resistance (turbidity in glass).
An alkali-free glass or a glass composition containing no alkali metal oxide, is disclosed in e.g. Patent Documents 1 to 13. However, the glass composition disclosed in the Patent Documents 1 and 2 has a small content of SiO2, whereby the acid resistance is insufficient. Further, the content of B2O3 is large, whereby the acid resistance is poor and the Young's modulus is also low.
The glass composition disclosed in each of Patent Documents 3 and 4 contains BaO as the main alkaline earth metal oxide in its composition, whereby the density of glass to be produced becomes large.
Further, the alkali-free glass disclosed in Examples in each of Patent Documents 7 and 8, is one having poor properties as a glass substrate for display devices. For example, the glass disclosed in each of Examples 20, 28, 29 and 42 contains Al2O3 in a large amount, and therefore the BHF resistance and devitrification property are poor.
As the alkali-free glass disclosed in Patent Document 9, for example, the glass in each of Examples 12 and 17 contains Al2O3 in a large amount, and it tends to be disadvantageous with respect to the BHF resistance and devitrification property.
Further, also as the alkali-free glass substrate disclosed in Patent Document 10, one having poor properties as a glass substrate for display devices is disclosed in Examples. For example, the glass in each of Examples 1 to 9 and Example 12 contains B2O3 in a small amount, whereby the BHF resistance and devitrification property are poor.
Further, as the alkali-free glass disclosed in Patent Document 11, one having poor properties as a glass substrate for display devices is disclosed in Examples. For example, the glass in each of Examples 4 to 7 has B2O3 in a large amount, whereby the acid resistance is poor and the Young's modulus is also low.
Further, as the alkali-free glass disclosed in Patent Document 12, one having poor properties as a glass substrate for display devices is disclosed in Examples. For example, the glass disclosed as sample No. 12 contains SiO2 in a small amount, whereby the acid resistance is poor.
Further, in any of Patent Documents 1 to 13, there is no description relating to the reducing resistance of alkali-free glass. Namely, in the prior art, no studies have been conducted at all on improvement of the reducing resistance of an alkali-free glass in order to solve the above problems occurred at the time of forming by a float process.
Patent Document 1: JP-A-6-56469
Patent Document 2: U.S. Pat. No. 5,506,180
Patent Document 3: JP-A-7-300336
Patent Document 4: EP 0,559,389
Patent Document 5: JP-A-11-157869
Patent Document 6: U.S. Pat. No. 6,096,670
Patent Document 7: JP-A-8-109037
Patent Document 8: U.S. Pat. No. 5,508,237
Patent Document 9: JP-A-9-169539
Patent Document 10: JP-A-9-156953
Patent Document 11: JP-A-4-325436
Patent Document 12: JP-A-2000-159541
Patent Document 13: JP-A-2001-220173